This is a continuation of studies of the pathophysiology of the circulation of the distal segment of the optic nerve. The tissue oxygen tension in the prelaminar optic disk and in the outer retina and the fovea centralis will be measured polarographically. The effect of changes in the systemic blood pressure, sympathetic nerve stimulation, and sympathectomy will be examined. The blood flow in the prelaminar optic disk will be measured by the method of hydrogen clearance. The effect on optic disk blood flow of changes in the perfusion pressure by changing the intraocular pressure and by changing the systemic blood pressure will be studied. The effect on the optic disk blood flow of sympathetic stimulation, and sympathectomy will be examined. BIBLIOGRAPHIC REFERENCES: Ernest, J.T., Stern, W.H. and Archer, D.B. (1976). Submacular choroidal circulation. Am. J. Ophth. 81:574. (Reprint in appendix). Ernest, J.T. (accepted June 1976). Optic Disk Oxygen Tension. Exp. Eye Res.